In the first part of the 20th century, engineers faced a problem commonly called the “Tyranny of Numbers”: there is a practical limit to the complexity of macroscopically assembled systems. Using discrete components such as vacuum tubes, complex circuits quickly became very expensive to build and operate. The ENIAC I, created at the university of Pennsylvania in 1946, consisted of 19,000 vacuum tubes, weighed thirty tons, and used 200 kilowatts of power. The transistor was invented at Bell Laboratories in 1947 and went on to replace the bulky vacuum tubes in circuits, but connectivity remained a problem.
Although engineers could in principle design increasingly complex circuits consisting of hundreds of thousands of transistors, each component within the circuit had to be hand-soldered: an expensive, labor-intensive process. Adding more components to the circuit decreased its reliability as even a single cold solder joint rendered the circuit useless.
In the late 1950s Kilby and Noyce solved the “Tyranny of Numbers” problem for electronics by inventing the integrated circuit. By fabricating all of the components out of a single semiconductor—initially germanium, then silicon—Kilby and Noyce created circuits consisting of transistors, capacitors, resistors and their corresponding interconnects in situ, eliminating the need for manual assembly. By the mid-1970s, improved technology led to the development of large scale integration (LSI): complex integrated circuits containing hundreds to thousands of individual components.
Microfluidics offers the possibility of solving similar system integration issues for biology and chemistry. For example, Unger et al., Science, 288 (5463): 113 (2000) previously presented a candidate plumbing technology that allows fabrication of chips with monolithic valves made from the silicone elastomer polydimethylsiloxane (PDMS).
However, devices to date have lacked a method for a high degree of integration, other than simple repetition. Microfluidic systems have been used to demonstrate a diverse array of biological applications, including biomolecular separations, enzymatic assays, polymerase chain reaction (PCR), and immunohybridization reactions.
While these are excellent individual examples of scaled down processes of laboratory techniques, they are also stand-alone functionalities, comparable to a single component within an integrated circuit. The current industrial approach to addressing true biological integration has come in the foam of enormous robotic fluidic workstations that take up entire laboratories and require considerable expense, space and labor, reminiscent of the macroscopic approach to circuits consisting of massive vacuum-tube based arrays in the early twentieth century.
Accordingly, there is a need in the art for high density, large scale integrated microfluidic devices, and methods for fabricating same